Laser processing of dielectric and conductive materials is commonly used to ablate fine features in electronic components. For example, chip-packaging substrates may be laser processed in order to route signals from the semiconductor die to a ball-grid array or similar package. Laser processed features may include signal traces, ground traces, and microvias (to connect signal traces between package layers).
Laser direct ablation (LDA) incorporates signal and ground traces on a single layer to tightly control signal impedance while reducing the number of layers in a chip package. Such an approach may require small feature dimensions and spacing (e.g., about 10 microns (μm) to about 25 μm), and long trace lengths per package (e.g., about 5 meters (m) to about 10 m). In order to construct chip packages economically, the speed at which such features are ablated may be quite high (e.g., from about 1 meter/second (m/s) to about 10 m/s). Certain packages may be processed, for example, in about 0.5 second (s) to about 5 s to meet customer throughput goals.
Another useful characteristic of chip packaging may be to provide intersecting traces with controlled depth variation. For example, ground traces may branch at several points throughout the pattern. At each branching intersection, the traces may be ablated with a desired depth variation of less than about +/−10%. Normally, if two trenches were to be ablated at one point, the double exposure of the ablating beam would create a depth variation of about 100%.
Another useful characteristic of chip packaging may be to provide variable trace widths at different parts of the package to control impedance or provide pads for inter-layer connection vias. Trace width control should be provided with reduced or minimal disruption to the high-velocity processing of the main trace.
It may also be useful to process features of arbitrary size and shape, at high speed, with reduced or minimal time used to change the feature's characteristics. For example, features may include microvias with a variety of diameters and/or sidewall taper, square or rectangular pads, alignment fiducials, and/or alphanumeric notation. Traditionally, for processing features such as microvias, optical systems have been designed to provide shaped intensity profiles (e.g., flat-top beams) of variable diameter, or purely Gaussian beams. These optical systems may have significant time delays (e.g., about 10 milliseconds (ms) to about 10 s) when changing laser processing spot characteristics.
Other problems are associated with building a machine to meet the processing parameters noted above. For example, traces may change direction throughout the package due to routing requirements. When processing traces at high velocity, the variation in trajectory angle may require high beam position acceleration at very short time scales. Laser processing can easily exceed the dynamic limits of the beam positioner, for example, when running at the high velocities (e.g., about 1 m/s to about 10 m/s) used for high throughput.
Such accelerations and/or velocities may be difficult to achieve in traditional laser processing machines, which have relied on beam-positioning technologies such as linear stages in combination with mirror galvanometer beam deflectors (referred to herein as “galvos” or “galvo minors”), along with static (or slowly varying) beam conditioning optics that cannot respond in the time scales used for this type of processing (e.g., on the order of about 1 microsecond (μsec) to about 100 μsec).
The actual ablation process may also be a factor to consider. Laser pulses with high peak power may be used to ablate the dielectric material while minimizing thermal side effects such as melting, cracking, and substrate damage. For example, ultrafast lasers with pulse durations (pulsewidths) in a range between about 20 picoseconds (ps) and about 50 ps at repetition rates of about 5 megahertz (MHz) to about 100 MHz can process materials with high peak power while providing significant pulse overlap to avoid pulse spacing effects. Fiber lasers now commonly provide pulsewidths in the nanosecond region at repetition rates of greater than about 500 kilohertz (kHz). Normally, for a given process condition (ablation depth and width), the “dosage” (power/velocity) applied to the processed material should be constant. However, at low velocities, the applied power may become so low that the peak pulse power may be insufficient to ablate the material without inducing thermal effects (e.g., melting and charring).
Beam positioner designs may deflect the process beam using galvos. The intensity profile of the process beam at a workpiece may be Gaussian (for simple focusing of a Gaussian beam), or a shaped intensity profile (e.g., flat-top profile) for beams conditioned by a fixed optic beam shaper.
The laser beam may generally be directed along a beam axis that is moved relative to a workpiece along a beam trajectory in a cutting direction along a cutting path to create a trench in the workpiece. Typically, laser pulses impinge the workpiece at spatially adjacent or overlapping locations along the beam trajectory. The spatially overlapping pulses may be sequentially generated by the laser system, or they may be delivered nonsequentially, such as described in U.S. Pat. Pub. No. 2010-0252959. The relative motion between the beam axis and the workpiece is typically continuous (to avoid throughput delays due to acceleration, deceleration, and settling time), and the beam axis is usually oriented so that it is perpendicular to the workpiece.
The laser pulses generally exhibit a cutting depth limit per laser pulse that is often a function of a variety of laser output parameters including, but not limited to, laser power, pulse repetition frequency, pulsewidth, wavelength, area of the laser spot impinging the workpiece, and material characteristics of the workpiece. To compensate for the cutting depth limit, the beam axis may be directed to locations along the trench multiple times to increase the total depth of the cut. The locations may be addressed multiple times by performing multiple passes of the laser beam axis along the beam trajectory, dwelling the beam axis over the locations, adjusting the speed of relative motion of the beam axis with respect to the workpiece, adjusting the spatial bite size (the nonoverlapping portion of the spot area of the laser pulse with respect to a nearest neighboring spot area), adjusting the temporal delay between the spatially adjacent or overlapping pulses, adjusting the depth of focus of the laser beam with respect the surface of the workpiece, and adjusting one or more of the previously noted other laser output parameters.
Despite control of all of these variables, trench formation is generally subject to taper. Taper refers to the shape and angle of the side walls of the trench. Side wall taper is significant because in many materials the taper generally limits the depth of the trench as a function of the width of the trench between the tops of the sidewalls at the surface of the workpiece. For example, when applying multiple laser pulses to a single location on a workpiece, the hole formed in the workpiece will increase in diameter as the depth of the hole is increased. The hole will have the widest diameter at the surface of the workpiece, and the side walls of the hole may taper such that the hole may have a negligible diameter at the bottom. In many instances, the depth per pulse diminishes after a rudimentary depth is reached, after which throughput diminishes and laser energy is wasted in the workpiece. Also in many instances, a damage threshold in the material in the workpiece may be reached before a maximum depth is reached by such a single-location, continuous-punching process.
One conventional solution for creating deep trenches involves limiting the amount of laser power continuously directed at a given location on the workpiece by directing the beam axis along the trench over multiple passes. Each pass of the beam axis deepens the trench. Some of the passes of the beam trajectory are also typically directed to be parallel to the intended midline of the trench, extending inwardly and/or outwardly, so as to increase the trench width between the side walls at surface of the workpiece. The increase of the trench width at the surface of the workpiece is employed to mitigate some of the depth challenges due to taper.
Unfortunately, extra passes of the beam axis along the beam trajectory (or parallel beam trajectories) require substantial increases in time devoted to making the trench (or throughcut) to the desired depth, especially when such beam trajectories cover long distances.
FIG. 1A is a plan view of a conventional group of beam trajectories 22, 24, 26, 28, and 30 along which a beam axis may travel to make a deep trench 20 in a workpiece 10. FIG. 1B is a graphical depiction of the number of passes that a beam axis may be directed along each of the beam trajectories 22, 24, 26, 28, and 30 shown in FIG. 1A to obtain a desired depth of the trench 20. FIG. 1C is a graphical depiction of the depth of the trenches along the beam trajectories 22, 24, 26, 28, and 30 shown in FIG. 1A. FIGS. 1A-1C demonstrate how the trench 20 may need to be widened at the top surface of the workpiece 10 to obtain the desired depth, increasing the processing time which adversely affects throughput.